The present invention relates to a method of producing preform parts by layering fibrous objects. The fibrous objects that are layered in this invention are nonwoven fiber mats produced, by a spray deposition process. The present invention is particularly well adapted for the use of near-net shape fiber mats, that is, mats whose relevant dimensions are quite close to the corresponding dimensions of the final product being manufactured. This significantly reduces the amount of cutting processes and corresponding material waste in the manufacture of the desired preforms and the final composite parts made from these preforms. The present invention is particularly advantageous in the manufacture of carbon-carbon brake discs due to the high raw material and processing cost for this type of material.
Composite materials are materials of choice in many industrial applications, including the application of carbon-carbon composites to brake components. In these carbon-carbon composite materials, fibers and matrix components are combined to provide a desirable material architecture. Features of microstructural design that can be varied include fiber length (short to continuous), reinforcement type (e.g., pitch or PAN, etc.), reinforcement architecture (woven or nonwoven), reinforcement configuration (planar or three-dimensional), and reinforcement orientation. In the manufacturing of carbon-carbon composite materials an initial preform is produced, which already includes the microstructural design of the final composite part. This preform is later densified by a series of techniques.
Two technologies of particular interest are the Programmable Powder Preform Process (“P4”) and Laminated Object Manufacturing (“LOM”).
The Programmable Powder Preform Process is a fully automated process for chopping and spraying fibers to produce a fiber mat. With appropriate automated control of the fiber deposition, the P4 process enables the creation of fiber mats with complex shapes of both defined outer contour and three-dimensional shape. The P4 process consists of two major steps. A computer controlled chopper head with rotation knives, generally mounted on a six-axis robot, is used to cut continuous fiber tow into segments of defined length. Fiber tow segments are then sprayed or dropped onto a planar or spatially contoured surface. The surface may be, for instance, a screen of perforated sheet metal. The screen is generally connected to a vacuum system, and the vacuum holds the fiber tow segments temporarily in place on the screen. A small amount of polymeric powder is applied to the fibers as binder. The next step is the consolidation of the mat by melting of the binder, such that the fibers are held in position and the mat can be manipulated. Once the mat is treated in this manner, it can be removed from the screen and is capable of holding its shape for further handling and processing. Current commercial P4 systems have the major disadvantage that they can be used only to manufacture thin-walled or shell-type composite structures—they cannot be used to make preform for thick composite parts. This disadvantage of current P4 processing is overcome by the present invention.
Laminated Object Manufacturing is used to fabricate parts by stacking material layers upon one another. Materials conventionally used in LOM are sheets of paper, green ceramic tape, or composite fabrics (prepregs). In currently used LOM processes, the required outline of the individual layers of the material being processed is cut (for example by a laser) from a continuous feed or from rectangular plates of the material. The cutting tool (e.g. the laser) then scribes the remaining material in each layer to a crosshatch pattern of small squares. The crosshatched part ultimately allows for removal of the final structure as the process is completed. An individual layer is adhered to the previously deposited layer by an adhesive on the backside of the layer. The application of LOM to composite manufacturing was addressed in Klosterman et al., “Interfacial Characteristics of Composites Fabricated by Laminated Object Manufacturing”, Composites Part A: Applied Science and Manufacturing, Vol. 29:9–10, pp 1165–1174 (1998), and in Tari et al., “Rapid Prototyping of Composite Parts Using Resin Transfer Molding and Laminated Object Manufacturing”, Composites Part A: Applied Science and Manufacturing, Vol. 29:5–6, pp. 651–666 (1998). See also U.S. Pat. Nos. 6,476,122; 6,585,930; 6,630,093; 6,655,481; and 6,660,209. Conventional flat parts are produced on a flat base plate, while curved composite laminates can be produced on shaped mandrels. Nevertheless, each individual layer is initially planar. In composite manufacturing with LOM, the draping characteristics of the composite intend for use is a factor determinative of its suitability for use in the LOM procedure.
LOM processes have major disadvantages for fabricating composite structures. The cutting process is an essential step in the process. This cutting process is inefficient in its use of material, and a large quantity of scrap material is produced. This scrap rate becomes unacceptable if material costs are to be reduced. Building structures with internal cavities is limited, as additional cutting and lift-up processes have to be included in such cases. The incorporation of gradients in material properties within the LOM is limited to a layering approach. That is, gradients can be achieved only across the stacking direction. The planar build technique does not allow for interconnection between the individual layers beyond the adhesive used to bond the individual layers. Finally, the LOM process is limited to the assembly of initially flat sheets of material. Curved layer object manufacturing as it exists today produces parts in which the curved layers are stacked onto a shaped mandrel. As a consequence all layers in the final product will then be in parallel to each other. The planar build technique does not allow for interconnection between the individual layers beyond the adhesive use to bond the individual layers. Such disadvantages of current LOM processes are overcome by the present invention.
The patent literature in the field of the present invention is well developed. U.S. Pat. No. 4,867,086 teaches stitching a continuous tow of structural yarn to a substrate in the preparation of multi-layer fiber/resin composites. U.S. Pat. No. 5,184,387 relates to needle punching in the production of multi-layer composites. U.S. Pat. No. 5,217,770 teaches structures that may include a plurality of helically wound braided tapes in the production of composites. U.S. Pat. No. 5,439,627 discloses a method of manufacturing composites which includes chopping a green tape or ribbon, mixing it with binder, and forming moldings from the mixture. U.S. Pat. No. 6,013,371 relates to the production of near net shape carbon pistons and other artifacts by sintering petroleum pitch powders, preferably with pitch-base carbon fibers. U.S. Pat. No. 6,365,257 teaches the fabrication of thick, three-dimensional preforms comprising fibers disposed as chords of a circle. U.S. Pat. No. 6,478,926 relates to the formation of structural preforms from electromagnetic energy-activated binder and reinforcing material. U.S. Pat. Nos. 5,654,059 and 5,705,008 disclose the fabrication of thick, three-dimensional structures comprising discontinuous themoset pitch fiber. Claim 4 of the ′008 patent recites “A method for producing a fibrous preform comprising the steps of: providing shaping means for holding fiber; placing discontinuous thermoset pitch fiber having a length greater than about 0.5 inch into said shaping means and forming a mat having a thickness in a range of from about ½ inch to about 4 inches and a density of from about 0.3 to about 0.6 g/cc; needle punching said mat at a needle density of from about 100 to about 10,000 per square inch to form filaments bundles and re-orient a portion of said filament bundles in the needled direction, thereby providing a three-dimensional filamentary structure; and carbonizing said filamentary structure by heating in an inert atmosphere to a temperature greater than about 1000° C. to provide a porous carbon preform”. All of these patents contain disclosure that is illustrative of the state of the relevant art and the knowledge available to those skilled in the art, and the disclosure of each of the above patents is hereby expressly incorporated by reference.
VARIABILITY IN COMPOSITE ARCHITECTURE. The present invention provides the ability to fabricate parts with a predefined internal composite architecture. Each individual layer used to build the composite preform should allow for an internal structure optimized for desired properties. This internal microstructure can be based on variations in fiber volume fraction, fiber type, fiber length, and orientation. This invention allows one to introduce such variations is material characteristics in targeted areas within the final three-dimensional product. With this invention, it is even possible to embed objects (dense material components of differing composition and properties) into the preform, if such is desired to provide enhanced properties.
NON-PLANAR MICROSTRUCTURES AND LAYER CONNECTIVITY. The present invention also provides an extension of the capabilities of Laminate Object Manufacturing to the fabrication of objects assembled of non-planar microstructures. Fibrous mats can be produced on contoured vacuum screens as well as on flat vacuum screens. Assembling such fibrous mats allows for the design of complex microstructures that are not achievable with other processes, and provides the capability for near-net shape. In carbon-carbon brake preforms, where interlaminar shear is an important factor, the interlocking of individual fiber mat layers can be accomplished. This can be achieved, for instance, by incorporating matching lugs and tabs into the individual layers, such that a form-fitting load transfer between layers is achieved. Furthermore, three-dimensionally shaped fiber mats can be used as components of the assembly.
NEAR-NET SHAPE. The process of the present invention, with its ability to assemble individual fibrous mats into a unitary preform having a desired shape, reduces or even eliminates cutting processes that are conventionally required to finish a preform. This significantly reduces material waste, as well as simplifying overall processing. Moreover, the lack of necessity for extensive cutting facilitates the manufacture of preforms for composite parts having internal cavities.
MANUFACTURE OF CARBON-CARBON COMPOSITES. The present invention can advantageously be applied to the manufacture of carbon-carbon composite brakes, where preforms made of carbon fibers in a pitch matrix represent an intermediate step in the production of the final product. In making such preforms with the present invention, pitch and polyacrylonitrile (PAN) fibers can be mixed, and fiber orientation can be adjusted to preferred load transfer conditions, especially in the lug areas and heat transfer directions. The out-of-plane direction introduced in this manner will also aid in heat transfer away from the friction surface. Locations near the friction surface can be optimized for wear and friction, while the remainder of the part can be optimized for heat storage and strength.